Now we have successfully completed the DB2 and SAP upgrade on Primary PRD server. So we need to reactivate HADR in PRD. Kindly suggest us how we can proceed further to activate the HADR in standby database.

In DB2 Database upgrade, SAP mentioned "

In an HADR environment, you can only upgrade the primary database. You cannot upgrade the standby database. Before the upgrade, you must execute the STOP HADR command on the primary and the standby database. After the upgrade, you have to re-create your standby database and initialize HADR again on the primary and the standby database."